SpaceX Dragon set for rendezvous with station

The SpaceX company's Dragon cargo capsule is expected to join up with the International Space Station (ISS) in the coming hours.

Nasa has cleared the unmanned vehicle to move in close to the orbiting platform after a successful series of demonstrations on Thursday.

SpaceX is attempting to become the first commercial concern to run a freight service to the ISS.

Dragon carries half a tonne of food and spares for the 400km-high laboratory.

Astronauts onboard the station will try to grab the ship with a robotic arm once it gets within reach.

The crewmen will then berth the vessel to a free port on the underside of the platform.

"Dragon's 'go' for berthing day," said SpaceX mission director, John Couluris, on completion of Thursday demos.

And Holly Ridings, the Nasa flight director overseeing the SpaceX mission, added: "We had a very successful day, a great day in space; and certainly from the Nasa side we're excited about how the mission is going so far."

Dragon was launched atop a Falcon 9 rocket from the Cape Canaveral Air Force Station in Florida on Tuesday.

It has spent the intervening period raising its orbit closer to the altitude of the ISS.

Thursday's tests required Dragon to show off its guidance, control and communications systems at a distance of just 2.5km from the station.

The operations were required by Nasa to prove the new vehicle's responsiveness.

Astronauts Don Petit and Andre Kuipers, reported seeing Dragon moving below them, and were even able to send a command to the capsule to turn on a strobe light.

Unlike the Russian and European robotic freighters that drive all the way into docking ports on the ISS, Dragon merely has to position itself in a zone 10m under the platform to be captured by the Canadarm2. It will be Petit who will be at the arm's controls to make the capture and berthing on Friday.

Ms Ridings, speaking at the Johnson Spaceflight Center in Houston, Texas, cautioned reporters that there was still much to be done: "This is a demonstration flight; a test flight, as we've been saying.

"To get through [Thursday] obviously makes you feel positive, but in terms of the activities for Friday, there's still a lot of really new things that the teams need to perform."

She stated that capture and berthing would probably occur around 10:30 CDT (15:30 GMT; 16:30 BST).

To date, only government-owned and operated vehicles have approached and docked with the space station.

The Dragon mission is part of Nasa's Commercial Orbital Transportation Services (Cots) programme which was established to help shift some of the agency's traditional roles and activities into the private sector.

The agency is providing seed funding of approximately $800m to SpaceX and another company, Orbital Sciences Corporation, to enable them to develop new rocket and cargo vehicles.

Orbital's rocket is called Antares, and its freighter is known as Cygnus. The pair should go into space together for the first time before the end of the year.

Once the companies have reached the milestones laid out under Cots, lucrative ISS re-supply contracts will kick in.

Watch Tuesday's launch from Cape Canaveral

For SpaceX, this is valued at $1.6bn (£1bn) and calls for a minimum of 12 Dragon missions to the ISS.

By buying in transportation services from the commercial sector, Nasa hopes to save money it can then spend on more challenging exploration ventures far beyond Earth, at asteroids and Mars.

Freight is just the first service to be contracted out. The ferrying of crews to and from the ISS will be next.

SpaceX wants this business as well, and is developing the safety and life-support systems that would allow Dragon to double up as an astronaut taxi.

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter